1. Field of the Invention
This invention pertains generally to ultrasonic diagnostic techniques, and more particularly to a method for assessing and displaying the three dimensional velocity magnitude of blood flow through an identified vessel using ultrasound.
2. Description of the Background Art
Diagnostic ultrasonic techniques in medicine are of growing importance, because they are non-invasive, non-ionizing, and of low cost as compared to other sensing and imaging methods. As a result, such techniques have been applied to assessing the integrity of the vascular system in a human body where it is necessary to determine the true magnitude of the flow velocity through the blood vessel. However, such a determination of the true three dimensional magnitude has not been possible using conventional ultrasound techniques because currently used strategies accurately measure velocity only in directions which are parallel to the beam; that is, radial or axial velocity, rather than transverse velocity.
As a result, various solutions for more accurate flow measurements have been proposed. For example, U.S. Pat. No. 4,265,126 issued to Papadofrangakis et al. on May 5, 1981, discloses using a cross-beam array configuration which is capable of resolving two orthogonal components of the velocity vector, the radial velocity component being resolved in the acoustic beamsteering direction and the transverse component being resolved at right angles to the beamsteering direction.
U.S. Pat. No. 5,000,184 issued to Bonnefous on Mar. 19, 1991, discloses measuring the axial and transverse components of the velocity in three dimensions using a mosaic of piezoelectric transducers.
U.S. Pat. No. 5,201,313 issued to Katakura on Apr. 13, 1993, discloses a flowmeter which utilizes an array of transducer elements to measure magnitude and velocity of flow, and further discloses a method of measuring the velocity at a right angle to an ultrasound beam by evaluating the flow rate in the transverse direction using the frequency spectrum of the detected signal.
U.S. Pat. No. 5,109,857 issued to Roundhill et al. on May 5, 1992, discloses that color flow doppler processors are not capable of measuring lateral or elevation vectors across the beam, and attempts to solve the problem by processing modulated signals returned from the test region to detect first and second ranges of data. The second range is compared to the first to determine positional displacement which indicates movement. From the movement data, velocity is determined.
U.S. Pat. No. 4,759,375 issued to Namekawa on Jul. 26, 1988, discloses use of ultrasonic pulse beams sent at different angles to determine radial velocity and tangential velocity. Tangential velocity is determined by obtaining first and second velocity distributions at different angles, and comparing the velocity distributions to obtain the vectorial velocity.
The foregoing approaches, however, are costly and difficult to use, thereby rendering conventional ultrasound techniques more practical. However, current systems are still based on using an estimate of the Doppler shift frequency to determine the axial velocity component, where the axial velocity is the velocity magnitude toward or away from the transducer. Therefore, such systems are unable to quantify the magnitude of any velocity component which crosses the transducer beam. As a result, these systems generally compensate for the absence of lateral information through one of two schemes: manipulation of the transducer to minimize the beam-vessel angle, thus maximizing the measured velocity component; or operator placement of a cursor in the direction of flow, based on the viewed image. In the latter case, the systems compute a two dimensional magnitude based on the direction of this cursor, but are unable to estimate motion in a direction into or out of the image.
Therefore, there is a need for a method for estimating the true three dimensional magnitude of blood velocity from ultrasound signals generated from conventional diagnostic equipment. The present invention satisfies that need, as well as others, and overcomes the deficiencies found in devices and techniques heretofore developed.
The foregoing patents reflect the state of the art of which the applicant is aware and are tendered with the view toward discharging applicant's acknowledged duty of candor in disclosing information which may be pertinent in the examination of this application. It is respectfully stipulated, however, that none of these patents teach or render obvious, singly or when considered in combination, applicant's claimed invention.